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PHOTO: Nataliya Vasilyeva/AP
Farkhanur Gavrilova, mother of Ruslan Gavrilov who is reported to have killed in Syria, looks on while speaking to the Associated Press in the village of Kedrovoye, Russia, Thursday, Feb. 15, 2018. Gavrilova's son Ruslan was one of seven men in this village of 2,300 who are believed to have joined a private military company called Wagner. The company reportedly was involved in an attack on U.S.-backed Kurdish fighters in Syria on Feb. 7 and suffered devastating losses in a U.S. counterstrike. (AP/Nataliya Vasilyeva)
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PHOTO: BART MAAT/AFP/Getty Images
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(CNN) —  

Pro-Syrian regime forces have begun massing east of the Euphrates River near where US troops are present while advising local allies, according to three US military officials. They are assembling near where similar pro-regime forces allied with Syrian President Bashar al-Assad initiated an unprovoked attack on US-backed Syrian Democratic Forces, the officials said.

One US military official said that the coalition had used the pre-established de-confliction hotline to raise the increased pro-regime presence with the Russian military.

The US military is monitoring the situation in Deir Ezzor to see if the pro-regime troops attempt to once again attack areas controlled by the SDF.

“While we do not seek confrontation with pro-regime forces, as we have demonstrated repeatedly, Coalition forces will not hesitate to protect themselves when they are threatened,” Pentagon spokesman Maj. Adrian Rankine-Galloway told CNN.

“Coalition forces maintain good situational awareness of military developments throughout Syria and take appropriate force protection measures to ensure our forces are safe and can carry out their combat missions to rid Syria of ISIS,” he added.

The US-led coalition said that the February attack by forces aligned with Assad was likely intended to seize nearby lucrative oil fields that US-backed forces had captured from ISIS.

During that attack pro-regime forces using tanks and artillery fired on a Syrian Democratic Forces headquarters where US Special Operations Forces were present. The US advisers called in for artillery and air support which included F-15E jets, MQ-9 drones, B-52 bombers, an AC-130 gunship and AH-64 Apache helicopters. Those strikes were estimated to have killed 100 pro-regime fighters, including several Russian military contractors according to US officials. A few days later the US-led coalition struck a pro-regime tank that was perceived to be threatening coalition forces in the same area.

The clash took place east of the “de-confliction line” which was established by the US and Russia to prevent accidental incidents by keeping US and Russian backed forces apart. The line largely runs along the Euphrates River.

Despite the significant losses suffered during their previous attempt to take the area, the pro-regime forces may believe that the Syrian Democratic Forces’ positions are now less well defended given recent news that SDF fighters have left the Middle Euphrates River Valley to join the fight against Turkish troops in the northwestern area of Afrin, Syria.

The departure of these forces has been publicly acknowledged by US and coalition military officials as well as the SDF.

The US-backed SDF announced on Tuesday that they will redeploy 1,700 fighters to fend off a Turkish offensive on Kurdish militias in Afrin.

A coalition official told CNN last week that these departures had already negatively impacted the ability of US-backed forces in the region to carry out major offensive operations against ISIS.